1. Field of the Invention
The present invention relates to devices for monitoring the concentration of an air/vapor mixture, and particularly for monitoring the concentration of solvent vapor in the fixing station of a non-mechanical printing or copying device.
2. Related Application
This application is related to the application of Hans Winter et al entitled Device for Monitoring the Concentration of an Air-Vapor Mixture, filed simultaneously herewith and identified with Ser. No. 360,434.
2. Description of the Prior Art
The use of solvent vapor to fix the toner images applied to a moving paper web in a fixing station of a non-mechanical printing or copying device is known from U.S. Pat. No. 3,049,810. The vapor, which may consist of Freon or methylene chloride, dissolves the toner so that it can penetrate into the paper. The fixing station in which the process takes place may consist, for example, of a container or housing including an evaporation location by means of which the solvent is converted from a liquid into a vapor. The paper web is conducted through the housing and is exposed to the solvent vapor inside the housing. A condensation trap is commonly attached in front of the container opening for preventing escape of solvent vapor into the environment.
A problem relating to the above known process and apparatus is that a perfect fixing of the toner images on the paper web can be achieved only when the solvent vapor has a specific concentration. It is therefore necessary that the concentration of the solvent vapor be determined within the fixing station by means of a measuring device and that the information obtained by the measuring device be employed to trigger an appropriate reaction if the concentration deviates from a desired level by an unacceptable amount.
Ultrasonic measuring devices operating on the principle that the transit time of an ultrasonic signal in a medium such as air varies depending upon the concentration of a vapor in the air are also known to those skilled in the art. Once such ultrasonic measuring device is known, for example, from German AS No. 2,024,882 in which the electroacoustical transducer serves the function of transmitting the ultrasonic oscillation as well as receiving the oscillation which has been reflected by a reflector.
It is also possible to transmit a brief ultrasonic oscillation and to measure the time between the transmission of the pulse and the arrival of the reflected received pulse by the echo sounding principle. A means is then required to generate and count small time intervals between transmission and reception of the pulse in order to determine the transit time. An electronic clock may be employed for generating a number of clock pulses at equal intervals which are counted by the use of, for example, a binary counter.
Ideally a device disposed in the fixing station of a printing or copying machine for providing an electronic signal corresponding to the concentration of the solvent vapor therein should provide a signal to an evaluation logic means connected to the sensing device for determining whether the vapor concentration is within the specified range for proper operation, whether new solvent must be supplied and evaporated, or whether deviations from the specified value are so large that the printing or copying machine must be shut off.
A problem which is associated with any vapor concentration monitoring means which is used to control the operation of a device is that temporary disruptions in the vapor concentration may occur within the volume of air monitored by the sensor which may result in a changing of the density of the fixing agent vapor which is temporally and/or topically limited and therefore is not representative of the average density of the solvent vapor in the entire volume of the fixing station. Such disruptions may be caused by, for example, the formation of eddys due to spontaneous evaporation when the solvent is injected into the fixing station, thermal stratifications between the heated evaporation location and the condensation trap, and turbulences due to the moving paper web. These usually brief and spatially limited disruptions of the homogeneous distribution of the solvent vapor mixture have only a negligible influence on the fixing quality. The minimally different effect of the solvent in the vicinity of such temporary disruptions is balanced out over the longer fixing path.
Such disruptions do, however, significantly vitiate the measurements of the vapor concentration and the evaluation thereof in the limited volume of conventional detection devices. Diffractions of the ultrasonic wave may be caused which result in interference patterns, particularly due to the topical fluctuations of the vapor concentration. Such interference patterns may partially or entirely cancel the echo from the ultrasonic reflector or prevent the generation of useable electrical pulses at the receiver due to out-of-phase excitation of various portions of the soundabsorbing surface of the ultrasonic transducer.